Why is "a" missing in "running like clockwork"
I would like to know why the idiom "running like clockwork" is not written as "running like a clockwork"? Is there some common rule that explains this?
I am asking this question since I am not sure that this word is a mass noun, see here: http://en.wikipedia.org/wiki/Clockwork
'Clockwork' is a mass noun, like 'water'. A clock does not have 'a clockwork' is just has 'clockwork' (like a river has 'water' not 'a water').
Clockwork is a noun that doesn't have to be instantiated in order to be used. Other examples of this include: "Slow as molasses", "Smooth as ice".